The present invention relates to an apparatus for controlling the rotational speed of an internal combustion engine, more particularly to an apparatus for controlling the engine speed to that suitable for auto-cruising.
For maintaining the speed of a vehicle at a predetermined level, there has, for example, been used an apparatus in which the fuel supplied to the engine is regulated by a closed control loop to reduce the difference between the actual engine speed and the target engine speed. In such a conventional apparatus, the rate at which said difference is reduced remains constant regardless of the magnitude of the difference. This leads to the disadvantage that correction overshoot is liable to occur when the difference is small and the disadvantage that a long time is needed for converging the actual and target speeds when the difference is large. To eliminate these disadvantages of the conventional device, Japanese Patent Public Disclosure No. 57-110745 proposes an engine speed controlling device wherein the loop gain of the control system is changed stepwisely as the amount of difference changes.
However, an offset condition will occur in this apparatus when the control state is changed from one control region into another. As a result, for example, the response characteristics in the control operation will rapidly vary when the control state shifts from a noncontrolled region provided as a neutral zone into a control zone. That is, the control operation will become discontinuous due to the sharp change in the control rate, making it impossible to control the engine speed smoothly.